1. Field of the Invention
The present invention relates to an optical fiber cable and more particularly to an optical fiber cable which realizes a reduction in size of a core loading material having a spiral groove to accommodate on optical fiber core and a method for manufacturing an optical fiber cable to accurately accommodate the optical fiber core to the spiral groove thereof.
2. Description of the Related Art
A cross-sectional view of an optical fiber cable of the prior art is shown in FIG. 1. In FIG. 1, an optical fiber core 1, which is formed by providing an optical fiber line consisting of a clad glass having the refractive index lower than that of a core glass and a silicon rubber layer covering the outside of the clad glass to the circumference of the core glass used as an optical transmission path and moreover covering and protecting outside of the fiber line with an outer sheath of a synthetic resin such as NYLON (trademark), polyethylene or polycarbonate, is set to about 0.5 mm to 0.9 mm in its outer diameter. This optical fiber core 1 is necessary to be protected from tension, side pressure or bending. In case several optical fiber cores 1 are bundled into a cable constitution, as shown in FIG. 2, such optical fiber cores are housed in the recessed grooves formed at the external circumference of the core loading material 3 made of molded synthetic resin such as NYLON, polycarbonate or other suitable material formed on the circumference of a tension member 2 which is preferably a steel wire or a fiber reinforcing synthetic resin rod material (FRP) coaxial with the core loading material. The recessed grooves are formed as spiral grooves 4 in the longitudinal direction. In FIG. 1 and FIG. 2, a total of eight (8) grooves are formed and an optical fiber core 1 is provided in three of the grooves. The number of optical fiber cores 1 and spiral grooves 4 may be set adequately. The external circumference of core loading material 3 housing the optical fiber cores 1 within the spiral grooves 4 is wound spirally in several layers by a tape 5 made of a synthetic resin such as NYLON and polyester and the external circumference is then covered with an outer sheath 6 made of synthetic resin such as vinyl chloride or polyurethane in order to protect the interior from environmental and mechanical influence. An optical fiber cable 7 is thus formed.
However, according to the optical fiber cable 7 of the prior art, a synthetic resin mold is formed by extrusion molding in order to constitute the spiral grooves 4 to accommodate optical fiber cores 1, resulting a problem that it is necessary to provide a large diameter, high tension member at the center in order to give strength to overcome the tension when it is laid to such mold. Thus, the outer diamond of optical fiber cable 7 becomes large as a result and thereby the weight per unit of length becomes heavier.
In addition, the prior art also has a problem that since the bottom part of spiral groove 4 is formed as a square shape for the convenience of manufacture, adequate clearance to the adjacent spiral groove 4 is necessary. The outer diameter of optical fiber cable 7 thus becomes large because the external pressure to be applied on the optical fiber core 1 is prevented by providing a margin to the spiral groove 4 for the optical fiber core 1.
The prior method of manufacturing optical fiber cable 7 will be explained with reference to FIG. 3. A long length core loading material 3 comprising therein a tension member 2 is wound around a cylindrical core loading material supply reel 8 having disk type edges at both ends. The end part of the core material 3 is released therefrom, transferred and moved linearly and is wound up by a take up reel 9 in the other side. In the linear part of core loading material 3 between the core loading material supply reel 8 and takeup reel 9, an optical fiber core loading apparatus 10 is disposed, along with a tape winding apparatus 11 which winds the tape around the core loading material loading optical fiber cores in the moving direction. In the optical fiber loading apparatus 10, the main body 13 rotatably driven by a motor 12 is provided with a plurality of arms 14. The end part of arm 14 is respectively rotatably provided with a core reel 15 would by a long length optical fiber core 1.
The optical fiber cores 1 are released from the respective core reels 15 and are loaded to the spiral grooves 4 of the core loading material 3. In FIG. 3 the spiral grooves 4 are not illustrated but these spiral grooves are formed in the condition as shown in FIG. 2. Concerning the torsional pitch of a spiral groove 4, a revolving gate depending on the torsion rate is given to the body 13 of the optical fiber core loading apparatus 10. The core reel 15 loads the core into the spiral groove 4 of the core loading material 3 while supplying the optical fiber core 1 by rotating around the circumference of core loading material 3.
The tape winding apparatus 11 located forward of the optical fiber core 1 loading position accommodates and protects the loaded optical fiber cores 1 so that the optical fiber cores 1 loaded cannot escape therefrom by densely and spirally winding the external circumference of core loading material 3 with the tape 5 of synthetic resin such as NYLON and polyester supplied from the tape reel 6. The reel is driven by a motor rotatably driving the rotating apparatus (not illustrated) to revolve around the core loading material 3. The outer sheath of cable 6 can be covered in the next process.
The method of manufacturing optical fiber cable of the present invention addresses a problem that the optical fiber cores 1 are not accurately loaded to the spiral grooves 4 and escape therefrom due to the mutual fine change of speed and torsional variation of core loading material 3 by the winding resistance force of tape 5. The moving speed of core loading material 3 and rotating rate for loading the optical fiber cores 1 to the spiral grooves 4 are detected and controlled so that they mutually match.
The problems described above tend to become more distinctive when the diameter of optical fiber cable 7 becomes smaller.